py3: avoid another b''.format() in chistedit
Now you'll get a different traceback if you try to confirm you
histedit plan.
Differential Revision: https://phab.mercurial-scm.org/D7184
To run the tests, do:cd tests/python run-tests.pySee https://mercurial-scm.org/wiki/WritingTests formore information on writing tests.